The present invention relates to a method of selecting bulk wood units for chemical pulping under alkaline conditions. As used herein, the term xe2x80x9cbulk wood unitsxe2x80x9d refers to logs or log segments or large planks of wood; the method of the present invention is designed predominantly for use in selecting or classifying unsawn logs, but it could also be used for selecting or classifying log segments or large sawn planks. As used herein, the term xe2x80x9cchemical pulpingxe2x80x9d includes semi-chemical pulping, i.e. processes in which wood is chemically pre-treated in a manner similar to chemical pulping, and then mechanically pulped.
The chemical pulping of wood to produce pulp for papermaking may be carried out by a number of different known techniques: the present invention relates specifically to those techniques which involve digesting the chipped wood in a bath of digester fluid which is alkaline based. The digester process generally is carried out at an elevated temperature and pressure.
The object of all of the digestion processes is to dissolve the lignin in the wood to release the individual fibres, leaving the cellulose and alkali-resistant hemicelluloses in the fibre walls. A typical wood sample has about 30 percent lignin, and this is reduced during the digestion processes to about 5%; the remaining 5% is removed from the pulp by bleaching. The lignin in the wood glues the fibres together and infiltrates between the cellulose and other constituents of the wood. It follows that the higher the percentage of cellulose and alkali-resistant hemicelluloses in the wood, the less lignin there is to be dissolved during the digestion stages, and the less bleaching is required; thus, the lower the percentage of lignin, (or the higher the percentage of cellulose) the lower the process costs. Hereinafter, the term xe2x80x98cellulosexe2x80x99 is taken to include both cellulose and residual alkali-resistant hemicellulose.
The most commonly used chemical pulping process is the sulphate or Kraft process, in which the wood chips are cooked in a mixture of caustic soda and sodium sulphide. The method of the present invention has been developed with the special reference to the Kraft process and therefore will be described with particular reference to this process. However, it will be appreciated that the method of the present invention also may be applied to select wood for any of the alkaline-based chemical or semi chemical pulping processes, (i.e. where the pH  greater than 7).
It is well established in the industry that some wood has a higher cellulose content, and therefore would be more efficient to process by chemical pulping. However, identifying which wood has a higher cellulose content simply cannot be achieved using current log sorting methods.
The traditional method of sorting trees at the point of harvest of the log is to categories and grade logs according to their diameter, length, straightness, diameter eccentricity and visual defects; the logs are placed in categories which reflect log diameter, log size and log grade. The basic assumption is that logs in each category are substantially identical. However, so far as chemical pulping yield is concerned, logs sorted in the above manner often prove to be far from identical, and may vary widely in cellulose content.
When a batch of logs is being processed by chemical pulping, it is of considerable economic advantage if all the logs have a similar cellulose content, since this will directly affect processing time and the quantities of processing chemicals required.
There is known to be a relationship between acoustic velocity through a bulk wood unit and its stiffness or modulus of elasticity. U.S. Pat. No. 6,026,689 discloses a system for predicting the modulus of elasticity of a bulk wood unit by generating a stress wave along the length of the unit by striking the unit (e.g. with a hammer), picking up vibrational signals from a standing stress wave in this unit, and using this information to calculate the speed of the stress wave in the unit, and hence the predicted modulus of elasticity for that unit.
It also is known that there is a relationship between the modulus of elasticity of a bulk wood unit and the microfibril angle, i.e. the angle of inclination of the stiff bundles of cellulose chains (microfibrils) which are embedded within the cell walls of the wood tissue. Generally, the microfibril angle is taken to refer to the helical inclination of the cellulose in the S 2 layer of the cell wall. (Page, D H, El-Hosseiny F, Winkler K and Lancaster A F 1877 xe2x80x98Elastic Modules of Single Pulp Fibresxe2x80x99 Tappi 60 (4) V 1-4 and Cave I.D. 1988 xe2x80x98The Anisotropic Elasticity of the Plant Cell Wallxe2x80x99 Wood Sciences and Technology 2 (4) 168-78).
In the paper by R. H. Newman (University of Canterbury Wood Technology Workshop of 1996), there was shown to be an empirical correlation between the modulus of elasticity of wood and its xe2x80x98purexe2x80x99 cellulose content (i.e. excludes hemicellulose), but the two properties were not shown to be derived from or directly dependent upon each other. In the development of the method of the present invention, it was postulated that there may be a direct relationship between the microfibril angle and the cellulose content of wood, although such a direct relationship has not yet been proved.
An object of the present invention is the provision is the provision of a method whereby a batch of logs may be reliably and accurately graded according to their likely yield during chemical pulping under alkaline conditions, by utilising the assumption that there is a sufficient relationship between the microfibril angle of wood and the cellulose content of that wood to permit cellulose content (and hence pulping yield) to be predicted from a measurement of acoustic velocity through the wood.
The present invention provides a method for sorting a batch of bulk wood units for chemical pulping under alkaline conditions comprising the steps of:
1) establishing a reference scale for the timber group to be sorted by:
a) selecting at random a plurality of sample units of bulk wood from the timber group;
b) measuring the acoustic velocity through each of said sample units using a predetermined measuring technique;
c) recording said acoustic velocities and grouping said velocities into two or more velocity bands;
d) processing all or part of each of said sample units to pulp using a predetermined chemical pulping process;
e) determining the pulp yield from each sample;
f) producing a reference scale indicating predicted pulp yield for a range of acoustic velocities;
2) measuring the acoustic velocity through each of said bulk wood units in turn, using said predetermined measuring technique;
3) comparing said acoustic velocity measurements against the reference scale to predict the chemical pulping yield for each tested unit; and
4) dividing the tested units into subgroups according to the predicted chemical pulping yield.
Preferably, before said acoustic velocity bands are selected, the acoustic velocities from all of said sample units are graphed to show the distribution of acoustic velocity in the total sample, to enable velocity bands to be selected such that a predetermined proportion of bulk wood units fall within each of the selected velocity bands.
Preferably, each of the batch of bulk wood units would be of the same or a similar species and would have a similar history i.e. each of the bulk wood units would be of a similar age, have been grown under similar conditions, and managed in a similar fashion.